“Can you make sure the dog’s water dish is full before you go to bed?”
“Make sure you write your name on your homework, or I won’t know whose it is!”

To make sure of something is to check that it is done. To make sure you do something is to do it yourself, without forgetting or ignoring it. When it’s important that something be done, we make sure it’s done, or tell someone else to make sure.

“I know it’s upsetting when you break up with your girlfriend, but don’t worry too much. There are plenty of fish in the sea.”

We say that there are plenty of fish in the sea to remind people that there are a lot of people in the world. There are so many people that there are always new people to meet, and with so many, some of them must be good matches. We use this phrase to comfort people who have broken up, been dumped or have trouble meeting a good match.

“She says she doesn’t care about him, but I can tell she’s got it bad.”

Sometimes “bad” can be used as an intensifier for something that isn’t necessarily bad. An intense desire can feel uncomfortable, and love is often compared to a disease, so that may be why we use this negative word to show their intensity.